It seems that a person can also click to select an object on the board, and then use the "zoom to selection" button near the bottom-right of the viewport, to bring things up to a more-visible size and place [some of] them on-screen.

Vertical panning is then accomplished by rolling the scrollwheel. Horizontal scrolling can be accomplished by tilting the scrollwheel on a mouse that is so-equipped.

(Perhaps bizarrely, I was unable to discover any other way to pan.)